Tachyon: The purely speculative particle that is supposed to travel faster than light. According to Sir Einstein's equations of special relativity, a particle with imaginary rest mass and a velocity more than c would contain a real momentum and energy. Ironically, the bigger the kinetic energy of a tachyon, the slower it travels, approaching c asymptotically (from the above) as its energy approaches infinity. On the other hand, a tachyon trailing kinetic energy travels faster and faster, till the kinetic energy approaches to zero (0), the speed of the tachyon approaches to infinity; such a tachyon with zero energy and infinite speed is termed as transcendent.
The special relativity does not appear to specifically keep out tachyons, so long as they do not cross the light-speed barrier and do not interrelate with the other particles to cause causality violations. The quantum mechanical examines of tachyons point out that even although they travel faster than light they would not be able to take information faster than light, therefore failing to violate the causality. However in this situation, when tachyons are by their very nature undetectable, it fetches into question how real they may be.